1. Field of the Invention
The present invention relates to a method for driving capacitive display device such as an inorganic EL display device and a simple matrix type liquid crystal display device or the like.
2. Description of the Related Art
Recently, a variety of plane display devices have been developed. In various kinds of the plane display devices, even when the materials for display elements and the voltage values applied to display panels are different from each other, the structures of the periphery voltage applying circuits and the periphery control circuits are similar to each other. Therefore, hereinafter, although a description will be made while taking an inorganic EL display device as an example in which a phenomenon of electroluminescence (hereinafter, referred to as xe2x80x9cELxe2x80x9d for short) is utilized, it is noted that the invention is not limited thereto.
FIG. 7 illustrates a basic structure of an inorganic EL display panel. In the inorganic EL display panel, disposed parallelly on an electrical insulating substrate such as a glass substrate 1 is a ribbon-like first electrode group 2, laminated thereon is a dielectric material 3, further laminated thereon is an EL layer 4, still further laminated thereon is a dielectric material 5 so as to form a three-layered structure. On the three-layered structure, disposed parallelly is a ribbon-like second electrode group 6 that extends in the direction crossing at right angles to the first electrode group 2 so as to form inorganic EL elements at the crossing portions between the first electrode group 2 and the second electrode group 6. The inorganic EL elements are formed in a space where the ribbon-like electrodes of the first electrode group 2 face the ribbon-like electrodes of the second electrode group 6, and between the electrodes at both sides are interposed a three-layered dielectric material layer comprised of dielectric material 3, 5 and an EL layer 4. Accordingly, the inorganic EL elements are capacitive and are possible to be handled as capacitors in an electrical aspect. In a simple matrix type liquid crystal display panel also, since between the electrodes are interposed an electrical insulating liquid crystal layer, the same as the inorganic EL element, the liquid crystal display element is possible to be handled as a capacitor.
FIG. 8 shows an example of relationship between an impressed voltage applied to an inorganic EL element and a light-emitting luminance thereof. The inorganic EL element emits light when a voltage is applied of which absolute value is larger than a light emitting start voltage of approximately 180V. The light-emitting voltage at which a sufficient luminance is obtained is approximately 230V. Further, in the inorganic EL element, since the characteristic thereof may be changed when a voltage of an identical polarity is applied for a long period of time, it is necessary to carry out an alternating current drive in which positive voltage and negative voltage are applied alternately. Consequently, for an inorganic EL element having a character as shown in FIG. 8, it is necessary to drive the same by means of a relatively high voltage of approximately xc2x1200V.
In an inorganic EL display panel having a basic structure as shown in FIG. 7, one of the first electrode group 2 and the second electrode group 6 of the inorganic EL elements is determined as a data-side electrode; the another is determined as a scan-side electrode. The inorganic EL elements formed at the crossing portions between the data-side electrodes and the scan-side electrodes constitute pixels respectively. Accordingly, in an inorganic EL display panel as shown in FIG. 7, the pixels are arrayed in a matrix configuration.
Conventionally, in a display device in which inorganic EL elements having a basic structure as shown in FIG. 7 are used, a scan-side drive IC is provided as a drive circuit of the scan-side electrodes; a data-side drive IC is provided as a drive circuit of the data-side electrodes. The scan-side drive IC includes a switching element for applying negative voltage to the data-side electrodes and a switching element for applying positive voltage thereto. The data-side drive IC is comprised of a structure in which a switching element for charging the EL layer 4 with modulating voltage, a switching element for discharging the same and diodes disposed in an inverse direction of the current of the respective switching elements are connected with each other. At the data-side, in accordance with display data, a modulating drive is made using the charging or discharging switching element while at the scan-side, the field that provides positive voltage and the field that provides negative voltage are repeated alternately to carry out, what is called, a field inverting drive in order to apply well-symmetric alternating pulses to the EL layer 4 providing a highly reliable display.
Furthermore, in a modulating system drive circuit for carrying out modulating drive in accordance with the display data, it is possible to use a driving element comprised of a double-well structured IC that has been recently developed. The modulating voltage applied from the data-side in accordance with the display data is modulated into a positive or negative voltage whereby a driving method is made possible in which the voltage that has the same absolute value is applied in both cases where a negative voltage or a positive voltage is applied to the data-side electrode as the driving voltage of the scan-side electrode. By virtue of a bipolar drive as described above, it is made possible to apply a better-symmetric alternating pulse voltage to the inorganic EL element.
FIG. 9 shows an equivalent circuit as a conventional modulating system drive circuit in which an inorganic EL element as shown in FIG. 7 is handled as just a capacitor having a capacity C as a capacitive display element 10. The capacitive display element 10 is driven by being charged/discharged at a voltage of +VM from a positive power supply 11 or at a voltage of xe2x88x92VM from a negative power supply 12. Current i that flows while charging/discharging the capacitive display element 10 from the positive power supply 11 or the negative power supply 12 flows also through a resistance 13 residing in the wirings or the like. The modulating system drive circuit may be represented as switches 21, 22 that switch the positive power supply 11 or the negative power supply 12 to apply a voltage to the capacitive display element 10.
FIG. 10 shows a driving voltage waveform in the equivalent circuit in FIG. 9. Now, assuming that a switch 21 represented with SWP is ON; a switch 22 represented with SWN is OFF, and the capacitive display element 10 is charged at a voltage VM. At this time, it is understandable that the potential level at the positive electrode of the capacitive display element 10 is +xc2xd VM, and the potential level at the negative electrode is xe2x88x92xc2xd VM. Now, a consideration will be given about a case where, from a state in which the capacitive display element 10 is charged with the switch 21 closed and the switch 22 opened, the same is charged at a voltage VM in inverted polarity by switching the switch 21 SWP to OFF; the switch 22 SWN to ON. The polarity of both electrodes of the capacitive display element 10 is inverted respectively, and electric power is consumed at a resistance 13 by the current i that flows at this time accompanying with the shift of the electric charge.
First, in case where a capacity C charged at a voltage xcex1 VM is charged at a voltage xcex2 VM, the electric energy consumed at a resistance R is calculated by Expression 1 as below. Herein, xcex1 and xcex2 are random values, respectively within xe2x88x921xe2x89xa6xcex1xe2x89xa61, xe2x88x921xe2x89xa6xcex2xe2x89xa61. At this time, the following Expression 1 is obtained.                               Ri          +                                    1              C                        ⁢                          ∫                              i                ⁢                                  ⅆ                  t                                                                    =                  β          ⁢                      xe2x80x83                    ⁢                      V            M                                              (        1        )            
When current i in Expression 1 is substituted with electric charge q, the following Expression 2 is obtained.                                           R            ⁢                                          ⅆ                q                                            ⅆ                t                                              +                                    1              C                        ⁢            q                          =                  β          ⁢                      xe2x80x83                    ⁢                      V            M                                              (        2        )            
General solution of a constant coefficient linear differential equation as shown in Expression 2 may be expressed as the following Expression 3 as an initial condition in which A is determined by a constant of integration.                               xe2x80x83                ⁢                  q          =                                    C              ⁢                              xe2x80x83                            ⁢              β              ⁢                              xe2x80x83                            ⁢                              V                M                                      +                          A              ⁢                              xe2x80x83                            ⁢                              ⅇ                                  -                                      t                    CR                                                                                                          (        3        )            
In the initial condition at t=0, being charged at a voltage xcex1VM, the following Expression 4 and Expression 5 are obtained from the Expression 3.                               C          ⁢                      xe2x80x83                    ⁢          α          ⁢                      xe2x80x83                    ⁢                      V            M                          =                              C            ⁢                          xe2x80x83                        ⁢            β            ⁢                          xe2x80x83                        ⁢                          V              M                                +                      A            ⁢                          xe2x80x83                        ⁢                          ⅇ                              -                                  0                  CR                                                                                        (        4        )            xe2x80x83∴A=C(xcex1xe2x88x92xcex2)VMxe2x80x83xe2x80x83(5)
By substituting Expression 5 for Expression 3, the following Expression 6 is obtained.                     q        =                              C            ⁢                          xe2x80x83                        ⁢            β            ⁢                          xe2x80x83                        ⁢                          V              M                                +                                    C              ⁡                              (                                  α                  -                  β                                )                                      ⁢                          V              M                        ⁢                          ⅇ                              -                                  t                  CR                                                                                        (        6        )            
By differentiating Expression 6 with time t to calculate current i, the following Expression 7 is obtained.                     i        =                                            ⅆ              q                                      ⅆ              t                                =                                                    C                ⁡                                  (                                      α                    -                    β                                    )                                            ⁢                              V                M                            ⁢                              ⅇ                                  -                                      t                    CR                                                              xc3x97                              (                                  -                                      1                    CR                                                  )                                      =                                                                                (                                          β                      -                      α                                        )                                    ⁢                                      V                    M                                                  R                            ⁢                              ⅇ                                  -                                      t                    CR                                                                                                          (        7        )            
Assuming that electric power consumed at the resistance 13 with a resistance value R is WR, the following Example 8 is obtained.                               W          R                =                              ∫            0            t                    ⁢                                    I              2                        ⁢            R            ⁢                          xe2x80x83                        ⁢                          ⅆ              t                                                          (        8        )            
By substituting Expression 7 for Expression 8, the following Expression 9 is obtained.                                                                         W                R                            =                                                                    ∫                    0                    t                                    ⁢                                                                                    (                                                                                                                                            (                                                                  β                                  -                                  α                                                                )                                                            ⁢                                                              V                                M                                                                                      R                                                    ⁢                                                      xe2x80x83                                                    ⁢                                                      ⅇ                                                          -                                                              t                                CR                                                                                                                                    )                                            2                                        ⁢                    R                    ⁢                                          ⅆ                      t                                                                      =                                                                                                    {                                                                              (                                                          β                              -                              α                                                        )                                                    ⁢                                                      V                            M                                                                          }                                            2                                        R                                    ⁢                                                            ∫                      0                      t                                        ⁢                                                                  ⅇ                                                  -                                                                                    2                              ⁢                              t                                                        CR                                                                                              ⁢                                              ⅆ                        t                                                                                                                                                                    =                                                                                          {                                                                        (                                                      β                            -                            α                                                    )                                                ⁢                                                  V                          M                                                                    }                                        2                                    R                                xc3x97                                  (                                      -                                          CR                      2                                                        )                                xc3x97                                                      [                                          ⅇ                                              -                                                                              2                            ⁢                            t                                                    CR                                                                                      ]                                    0                  t                                                                                                        =                                                1                  2                                ⁢                C                ⁢                                                      {                                                                  (                                                  β                          -                          α                                                )                                            ⁢                                              V                        M                                                              }                                    2                                ⁢                                  (                                      1                    -                                          ⅇ                                              -                                                                              2                            ⁢                            t                                                    CR                                                                                                      )                                                                                        (        9        )            
now, considering a limit of txe2x86x92∞, the following Expressing 10 is obtained.                               W          R                =                              1            2                    ⁢          C          ⁢                                    {                                                (                                      β                    -                    α                                    )                                ⁢                                  V                  M                                            }                        2                                              (        10        )            
Therefore, in case of charging at a voltage xe2x88x92VM in inverted polarity from a state being charged at a voltage VM, since xcex1=1, xcex2=xe2x88x921, by substituting them for Expression 10, the electric power consumption W1 by the resistance 13 of resistance value R, is expressed as the following Expression 11.                               W          1                =                                            1              2                        ⁢            C            ⁢                                          {                                                      (                                                                  -                        1                                            -                      1                                        )                                    ⁢                                      V                    M                                                  }                            2                                =                      2            ⁢                          CV              M              2                                                          (        11        )            
As for a technique for reducing loss electric power W1 like this, for example, Japanese Unexamined Patent Publication JP-A 6-35416 (1994) discloses a method for driving active matrix type liquid crystal display device, in which a voltage selected out of a plurality of voltage levels excluding the maximum voltage corresponding to the tone displays is applied with a voltage having an inverted polarity after the tone level is decreased once.
In an inorganic EL display panel having a basic structure as shown in FIG. 7, the electrode groups at the scan electrode side are simply selected and driven one by one. Whereas, as for the electrode groups at the data electrode side, it is necessary to drive every electrode groups every scanning operation. That is to say, in the modulating system drive circuit at the data electrode side, since the entire EL display panel is charged every scanning operation, the electrical power consumption becomes large as well as the ratio in the electrical power consumption of the entire display apparatus becomes large. Accordingly, in order to reduce the electrical consumption effectively, it is necessary to reduce loss electric power expressed in Expression 11 due to the resistance in the drive circuit.
As the driving method disclosed in the aforementioned JP-A 6-35416, a technique, in which a lower voltage level is selected out of a plurality of voltage levels provided according to the respective tone displays, and after converting the voltage level the voltage with an inverse polarity is applied thereto, is not applicable to a case where the drive circuit does not have a plurality of voltage levels, for example, a drive that does not carry out tone display or does not perform intermediate tone display; or, a drive in which intermediate tone display is performed by carrying out pulse width control of the voltage corresponding to the tone level, or the like. Also, since the electric charge charged in the capacitive display element 10 is entirely consumed by the resistance 13, the electric power consumption becomes large as well as a problem of a heat generation is also resulted in.
An object of the invention is to provide a method for driving a capacitive display apparatus, capable of reducing electric power consumption caused by electric current that flows to charge/discharge the capacitive display element during driving the same alternately by means of a voltage with positive and negative polarities.
The invention provides a method for driving a capacitive display apparatus having a plurality of display pixels comprised of capacitive elements which are disposed respectively at crossing portions between data-side electrodes and scan-side electrodes, the method comprising the steps of:
driving the data-side electrodes at a voltage corresponding to display data, while scanning the scan-side electrodes to be selected sequentially;
applying a driving voltage to the display pixels to carry out a display operation; and
when the data-side electrodes are driven to be charged with a voltage having an inverse polarity relative to a voltage charged at a previous charge, charging/discharging the data-side electrodes once to an intermediate potential level of which absolute value is smaller than the voltage applied for display operation between the charging voltages before and after driving the same.
According to the invention, in the capacitive display apparatus that includes a plurality of display pixels comprised of capacitive elements respectively disposed at the crossing portions between the data-side electrodes and the scan-side electrodes, display operation can be carried out by driving the data-side electrodes at a voltage corresponding to display data and applying a driving voltage to the display pixels while scanning sequentially scan-side electrodes to be selected. When driving the data-side electrode to charge the same at a voltage with an inverse polarity relative to the voltage charged at the previous charge, since the same is charge/discharged to an intermediate potential level of which absolute value is smaller than the voltage applied to display between the charging voltages before and after driving the same, it is made possible to reduce electric power consumption as compared with a directly charging/discharging between the charging voltages before and after driving the same. Because the electric power consumption is proportional to the square of the voltage, it becomes smaller when being modulated once to an intermediate potential level. Since the data-side electrodes are necessary to be driven every scanning operation, it is made possible that a reduction of electric power consumption for driving the data-side electrodes contributes largely to reduce electric power consumption for driving the entire capacitive display apparatus.
According to the invention, in the process of charging/discharging the capacitive display element, since an intermediate potential level is provided, after carrying out charging/discharging once using the potential as a power supply, and then a charging/discharging to the objective potential level from the present potential level is carried out, it is made possible to reduce electric power consumption largely. Particularly, as for the data-side electrodes, it is necessary to drive every electrode every time of scanning. Accordingly, since the frequency to charge/discharge the capacitive display element with a polarity different from each other before and after driving the same, it is made possible to make it contribute to reduce electric power consumption with a large ratio.
Further, in the invention it is preferable that when the scan-side electrodes are driven so as to be charged at a voltage with an inverse polarity relative to the voltage charged at the previous charge, the scan-side electrodes are charged/discharged once to an intermediate potential level between the charging voltages before and after driving the scan-side electrodes.
According to the invention, also when driving the scan-side electrodes at a voltage with an inverse polarity relative to the voltage charged at the previous charge, since the scan-side electrodes are charged/discharged once to an intermediate potential level between charging voltages before and after driving the same, it is made possible to reduce larger electric power consumption as compared with a direct charging/discharging between the charging voltages before and after driving the same.
Furthermore, according to the invention, when driving the scan-side electrode, since the potential is once charged/discharged to an intermediate potential level, it is made possible to reduce electrical power consumption.
Still further, in the invention it is preferable that the intermediate potential is the ground potential.
According to the invention, when charging/discharging the data-side electrodes or scan-side electrodes, as charging/discharging the same to the ground potential as the intermediate potential, in the case where charging/discharging is made between the voltages having the same absolute value but a polarity different from each other, by charging/discharging once the same to the ground potential, it is made possible to reduce electrical power consumption to a half.
Still further, according to the invention, since the ground potential is used as an intermediate potential, it is not necessary to provide additional power supply. Accordingly, it is made possible to build the structure at a low cost as well as to carry out charging/discharging so that electrical power consumption results in xc2xd as compared with the case where direct charging/discharging is carried out.
Still further, in the invention it is preferable that the charging/discharging to a intermediate potential is carried out in such manner that the power supply potential to be charged/discharged is gradually modulated in three or more steps within a range of the charging voltages before and after driving the scan-side electrodes.
According to the present invention, when charging/discharging, since the power supply potential is gradually charged/discharged in three or more steps, it is made possible to further reduce electrical power consumption.
Still further, according to the invention, since the charging/discharging to an intermediate potential is made while modulating the voltage gradually in three or more steps within the range between the voltages before and after charging/discharging the scan-side electrodes, it is made possible to further reduce electrical power consumption.
Still further, in the invention it is preferable that the charging/discharging to an intermediate potential is carried out using a capacitor of random potential between the charging/discharging voltages before and after the driving the scan-side electrodes.
According to the invention, when charging/discharging the voltage charged in the capacitive display element to a voltage with an inverse polarity, since the charging/discharging to the intermediate potential is carried out using a capacitor of random potential between the voltages before and after charging/discharging the scan-side electrodes, the voltage between the capacitive display element and the capacitor varies due to the charging/discharging to the capacitor. Accordingly, it is made possible to further reduce the electric power consumption.
Still further, according to the invention, when the charging/discharging is made, since a capacitor of random potential is used as a power supply for once charging/discharging to the intermediate potential, it is made possible to further reduce the electric power consumed when charging/discharging to the intermediate potential.
Still further, in the invention it is preferable that an electric charge collected to the capacitor of random potential is reused.
According to the invention, since the electric charge collected to the capacitor of random potential during charging/discharging the capacitive display element is reused, it is made possible to utilize effectively the electric power which is consumed as a loss electric power.
Still further, according to the invention, since the electric charge collected to the capacitor of random potential is reused, the electric power which, in the case where the capacitor is not used, should be consumed, is used effectively. Accordingly, it is made possible to reduce the entire electric power consumption.
Still further, in the invention it is preferable that the process of charging/discharging to the intermediate potential is carried out in such way that the charging/discharging to the capacitive display element is made, not by charging/discharging an electrical charge directly to the capacitive display element, but by charging/discharging an electrical potential of one electrode of the capacitor to change an electrical potential of another electrode.
According to the invention, it is made possible to charge/discharge the capacitive display element while collecting the electric charge to the capacitor, and to reuse the electric charge collected to the capacitor for driving the capacitive display element.
Still further, according to the invention, by connecting the capacitive display element with the capacitor in series, it is made possible to reduce electric power consumption as well as to reuse the electric charge charged in the capacitor.
Still further, in the invention it is preferable that the capacitive display element is an inorganic EL element.
According to the invention, it is made possible to reduce electric power consumption used for driving the inorganic EL element which handles a voltage of relatively high absolute value.
Still further, according to the invention, it is made possible to reduce electric power consumption in the case where the inorganic EL element is driven.
Still further, in the invention it is preferable that the capacitive display element is a liquid crystal display element.
According to the invention, it is made possible to reduce electric power consumption necessary for driving the liquid crystal display element.
Still further, according to the invention, it is made possible to reduce electric power consumption in case where the liquid crystal element is driven.